The bodyguard, Dan Clark, didn’t mean for it to hurt, he was just demonstrating a fighting move called the brachial stun. That’s a strike to the side of the neck using a chopping motion with the hands — but when Clark hit me with it in a slow speed demonstration, the sudden burst of force whipped my whole head to the right, crashing my teeth together so hard that I thought I would lose a filling.

That’s what Dan Clark is like — even his demonstrations are intense.

He’s the kind of guy who throws around phrases like the “muay Thai clench with a double knee strike,” and who, when he emails you to invite you to his training session, reminds you politely to bring a cup.

All of it — the gym, the training, and especially the punch — combine to send a clear message: Don’t mess with Buffett, or any of Clark’s other clients. And that’s kind of the point.

The gym we’re standing in is attached to the offices of his company, Clark International, in Omaha, Neb. Clark, with his square build and close cropped red hair, looks every inch the former Omaha police officer he once was.

But then he met Warren Buffett, the billionaire investor known as the Oracle of Omaha, and in 1995 became Buffett’s go-to guy for personal security. When you see Buffett walking the floor at his annual investor conference, Clark is usually positioned just to Buffett’s side — keeping the crowd from surging too close to the elderly billionaire, and making sure the camera crews don’t trample anybody.

Clark International training session

Such close proximity to an icon of American business inspired an entrepreneurial instinct in Clark, so he founded his own security company. Today, Clark International provides security services for high profile people including former Alaska Governor Sarah Palin and actor George Clooney, along with a list of others he declines to name.

We flew to Nebraska to spend some time with him this spring, during a two-day executive training session Clark had organized for dozens of full- and part-time security guards. Only a handful are full-time employees. The rest are contractors on call for big jobs that require more bodies on the ground.

The men — and one woman — clearly respect the man who’s training them, occasionally clobbering them, and who may just sign their next pay check. Two or three are veterans of the mixed martial arts fighting circuit. But even they call Clark “sir.”

Training with Dan Clark and his team

I’ve never even seen a machine gun fired in person before. But Clark’s team has access to the same outdoor gun range that the Omaha National Guard SWAT team uses, and they’ve rolled an old car onto the dirt strip in front of the targets.

Eamon Javers at the firing range

They’re working with M-4 carbines, which are a little shorter and lighter version of the military M-16. Those are handy features for security guards, because they train to be able to use them instantly from inside the front seat of a car, responding to an ambush attack so quickly that there’s no time to even get out of the car and take up a firing stance.

I’m sitting in the back seat as two of Clark’s men begin, blasting away with their machine guns from the front seat, through the windshield, and onto the targets. It’s hot, and its noisy. Shell casings fly out of the M-4s. One of them hits me in the head, and I’m a little surprised at how hot it is. When it’s all over, I count: one of the men fired 15 rounds, but with such a steady hand that there are only three holes in the windshield.

The training sessions start early and go late. Over the course of the two days, we watch as Clark’s team practices formations for guiding VIPs through crowds, conduct more weapons training and practice advanced driving techniques to avoid roadside ambushes. They let me fire a Glock .40, a pistol that is lighter in the hand than I thought but has a more powerful kick than I was expecting.

For the last exercise, they take turns deliberately smashing into a parked car in a maneuver designed escape a roadside trap. The technique involves crashing the right front of the escaping car into a spot just behind the right rear wheel well of the blocking car, which, if you do it at a high enough speed, causes the blocking car to spin out of the way. They do this again and again, switching drivers in the lead and follow-on cars each time so that everyone gets a chance to practice the move. CNBC producer Kelly Lin has affixed small portable cameras to both cars to record the moments of impact, and somehow they survive. But the lead car, an old white Cadillac STS, is rendered undrivable.

Clark’s men say they’ll bring it to the scrap yard the next day. The way my neck tightens up, I know just how that car feels.